To better understand how the force-producing myosin crossbridge interacts with actin, we have measured the crossbridge detachment rate-constants in rabbit skeletal muscle fibers and compared them with those for detachment of myosin subfragment-1 from actin in solution. We find that, in the fiber, crossbridge detachment is not described by a single rate- constant but by multiple rate-constants spanning a wide range. The fastest rate-constants are comparable to the detachment rate-constant of S1 from actin in solution but the majority are much slower. In FY '86 we showed that part, but not all, of the slowness in the apparent detachment rate-constants could be related to crossbridge clustering. In FY87 we examined how the rate-constant for crossbridge detachment in the presence of MgPPi depended upon MgPPi concentration. It was shown that if crossbridge heads act independently, the concentration dependence of the rate of crossbridge detachment should have the same form as the Michaelis-Menten equation. However, we found experimentally that the concentration dependence relationship was sigmoidal. The most likely explanation for our finding was that both of the S1 heads of a crossbridge need to bind MgPPi before either has a high probability of relaxing a substantial fraction of the tension it supports. This, in turn, provides an additional explanation for why the crossbridge detachment rate-constants in the fiber are smaller than the corresponding rate-constants in solution. If two heads of a crossbridge need to detach simultaneously before either head can relax the tension it supports, this process would indeed be expected to be slower than the rate at which S1 detaches from actin in solution. We also determined what fraction of time a crossbridge with bound ATP spends attached to actin in relaxed rabbit psoas fibers. For fibers that have has their sarcolemma made permeable, this number is 50-100% at very low ionic strength and 3-5% at physiological ionic strength. For intact fibers this number is estimated to be 5-10%.